1. Field of the Invention
The present invention relates to a step-down DC-DC converter, particularly to the step-down DC-DC converter which can take out stable voltage in a broad input-voltage range.
2. Description of the Related Art
The step-down DC-DC converter which converts an input voltage into a direct-current voltage lower than the input voltage by interrupting the input voltage with a switching element is well known. For example, in the step-down DC-DC converter disclosed in Japanese Patent Laid-Open Publication No. H5-91732 (document 1), an N-channel FET is used as the switching element, the voltage generated by energy accumulated in a reactor during a time interval in which the N-channel FET is turned off, is added to the input voltage and then the voltage is supplied to a PWM drive circuit which drives the N-channel FET.
Further, in the step-down DC-DC converter disclosed in Japanese Patent Laid-Open Publication No. H8-33322, in order to securely prevent over-voltage, a circuit which performs charging and discharging in response to the turn-on and turn-off of a P-channel FET as a semiconductor switch is provided, and its output is stopped when a charging voltage of the charging and discharging circuit indicates an abnormal value.
Thus, conventionally the N-channel FET or the P-channel FET is used as the switching element of the step-down DC-DC converter. When the N-channel FET is driven, because the voltage higher than a drain voltage which is of a power supply voltage is applied to a gate, a bootstrap circuit described in the document 1 is required.
On the other hand, the P-channel FET can be driven when the gate voltage is lowered with respect to the source voltage, so that a circuit configuration is relatively simplified.
However, due to the configuration of FET, when compared to the N-channel FET, it is difficult to produce the P-channel FET which has a good on-characteristic or a good switching characteristic, and the P-channel FET has tendencies of low withstand voltage and low current capacity. Therefore, the P-channel FET having the good characteristics cannot be produced.
Particularly the P-channel FET is not good enough when good operation is required in applications in which the input voltage fluctuates. For example, in the step-down DC-DC converter used for an engine generator which is started up using a recoil starter, it is necessary to start engine control in the low number of revolutions, and it is also necessary to securely operate to electric power generated by the high number of revolutions after the start-up. The input voltage to the step-down DC-DC converter fluctuates in the range of 5 to 450 volts (V). Therefore, in the circuit in which the N-channel FET is used, it is difficult to configure a control power supply of the engine generator based on the inputs of such the broad voltage range.
In order to take out the best output voltage of the generator from an early time when the DC-DC converter cannot output the maximum voltage because of the low number of revolutions, it is necessary to make FET in an on state (substantially 100% duty). However, in the conventional N-channel FET driving method, because the boostrap circuit is charged during the time interval in which FET is turned off, the turn-off interval of FET is required and FET cannot be driven at duty of 100%.